revisit how `font.fontExtension` is being set and handeled

[vladavoX]

font.fontExtension is undefined, type was incorrect, so I must add at least this if (font.fontExtension) {, but we can handle it better in new PR sure

Originally posted by @vladavoX in #112 (comment)

• Check if fontExtension exist when creating it
• Throw error right below when creating const fontExtension if doesn't exist